The present invention relates to a variable magnification optical system used in a copier or similar optical scanning device, and more particularly to a variable magnification optical system wherein a lens having a fixed focal length and a group of mirrors are moved to vary a projection magnification at the projection surface (i.e., a photoconductive drum).
A variable magnification optical system such as that in an electrophotographic copier has a lens for forming an image of an original on the projection surface, and a mirror group between the lens and the projection surface. Both of the lens and mirror group are moved in the optical axis direction to vary the distance between the surface of an original document and the lens, and to vary the distance between the lens and the projection surface, whereby both enlarging and reducing operations are performed. The lens and mirror group are mounted in respective movable members, and are accommodated in a housing and slidable in the direction of the optical axis of the lens. The housing shields non-image-forming light (i.e., except that in the optical axis direction), and the respective movable members are moved within moving ranges by a driving device in the housing.
Conventionally, the moving ranges of the lenses and mirror group are arranged in series in the optical axis direction, so that if the magnification range is increased, the length of the movement of the lens and mirrors increases. In this case, the space occupied by the magnification changing portions of the optical system is enlarged, and a lens having a long focal length becomes necessary. Accordingly, it is difficult to make the copier compact.
Light shielding members movable within the housing can further increase the size of the housing. That is, horizontal and movable flat plate light shielding members (for example, being made of metal) provided to shield the stray light that intrudes from above the housing when the lens is moved to different magnification positions, can interfere with the side walls housing, and the lens cannot reach all magnification positions. Accordingly, it becomes necessary to form an opening in the side walls of the housing for allowing the shielding member, or to make the housing longer in the optical axis direction. In order to allow the shielding member to protrude from the opening in the rear wall, it becomes necessary to have vacant space at the rear side of the housing, and it is thereby difficult to install parts in the rear side of the housing.
Still further, in a conventional variable magnification optical system, as a movable member supporting the lens is moved in the optical axis direction, a space is formed between the outer peripheral area of the moving member and the inner peripheral area of the housing. However, this space functions as a passage for stray light generated by diffuse reflection from the original document. Particularly, stray light passing through the space formed at both sides of the lens moving member can reach the projection surface via the mirror group, thereby causing flare and/or lack of contrast on the projected image.
In order to avoid this, it can be considered to provide a light shielding member for shielding stray light penetrating the space at the sides of the lens moving member. However, with this construction, when the lens moving member moves, it interferes with the light shielding member.